1. Field of the Invention
The present invention relates to a surface-acoustic-wave filter, and more particularly to a multi-transducer surface-acoustic-wave filter for providing a good filter characteristic capable of assuring a sufficient effective attenuation in an attenuation region but without deteriorating the filter characteristics in a pass band thereof. The surface-acoustic-wave filter is applicable to a band-pass filter in HF, VHF, and/or UHF bands.
2. Description of the Prior Art
Various types of surface-acoustic-wave filters have been proposed and put to practice up to now. FIG. 1 is a schematic plan view of a prior surface-acoustic-wave filter having two transducers (hereinafter referred to as a two-transducer filter), wherein each transducer is of an interdigital type which has interdigital electrodes. Element 1 is a piezo-electric substrate; element 2 is an input transducer; element 3 is an output transducer; element 4 is an input terminal; element 5 is an output terminal, and elements 6 and 7 are respectively surface-acoustic waves transduced in response to an input signal through the input transducer 2 and delivered.
Since the transducer is bidirectional in general, an electrical signal supplied to the input terminal 4 is transduced through the input transducer 2 equally with respect to two surface-acoustic-waves propagating in two opposite directions. The output transducer 3 accordingly receives only the surface-acoustic-wave propagating in one direction, i.e.--it receives 1/2 of the supplied electrical signal, whereas it delivers from the output terminal 5 only 1/2 of the received signal, i.e.--only 1/4 of the supplied electrical signal. It is accordingly known that a two-transducer type filter has an insertion loss of 6 dB, even if it is ideal.
To reduce the insertion loss of such a surface-acoustic-wave as described above, a surface-acoustic-wave filter, named a multi-transducer filter, has been proposed for use in various applications up to now.
The filter, as shown in FIG. 2, for example, comprises a structure such that a plurality of input and output transducers (five transducers in the illustrated case) 2 and 3 are alternately and equally spaced on the piezo-electrical substrate 1. The illustrated five-transducer filter is constructed such that the input transducer 2 transforms a supplied electrical signal into surface-acoustic-waves which are then received by a plurality of output transducers 3 whereby the energy of the surface-acoustic-waves which are not transformed into output signals due to their leakage to the outside of the surface-acoustic-wave filter and due to other reasons, can be reduced. The insertion loss of this type of filter can be further decreased as a number of the transducers is increased.
FIG. 3 shows a loss (dB) versus frequency characteristic of the five-transducer filter and that of the previously described two-transducer filter where fo is a center frequency thereof. It is evidenced from FIG. 3 that, in contrast with the two-transducer filter (as shown by a broken line I), the five-transducer filter (as shown by a solid line II), although being advantageous in a fact that the loss in the pass band is reduced, has drawbacks in that the loss in the attenuation region is sharply decreased and so as a difference between the minimum loss in the attenuation region and the maximum loss in the pass-band, that is, the effective attenuation, is reduced.
For example, the effective attenuations of the five-, seven-, and nine-transducer filters are respectively deteriorated by about 10, 15, and 20 dB as compared with that of the two-transducer filter.
The reason is as follows. Since a conventional multi-transducer filter has, as shown in FIG. 2, intercentral distances between the center of adjoining respective input and output transducers (i.e.--the propagation distances of a surface-acoustic-wave through the transducers) spaced in an equal interval l(i.e.--constant), electrical signals simultaneously applied thereto and delivered from the respective output transducers have the same phase in a certain frequency interval. Therefore, a gain of the multi-transducer filter in the attenuation frequency region has sharp peaks in a frequency period of V/2 l (where V is a propagation velocity of the surface-acoustic-wave).
FIG. 4 shows the filter characteristic, wherein the frequencies and gains (dB) are respectively plotted on the abscissa and ordinate axes, and wherein fo is a center frequency. As shown in the figures, the whole characteristic of the multi-transducer filter is deteriorated in its effective attenuation.